The present invention is in the field of fire alarm and detection. Early examples of prior systems of this general type may be appreciated by reference to following U.S. Pat. Nos. 4,568,919, 4,752,698, 4,850,018, 4,954,809, 4,962,368.
Most of the above cited U.S. patents describe systems that are approximately six to ten years old, and in most of these systems the loop controller, or the like, initiates the determination of the states of the units at the various zones or stations in the system by the use of a repetitive polling scheme for polling the detector units or stations from the loop controller, whereby addresses are sent successively on the loop or lines to determine which, if any, units are in an alarmstate. Provision is also made in most of these systems to detect trouble conditions in the system.
Other fire detector and alarmsystems have been developed in the recent past, that is, in the past five years or so, that provide a variety of features, including the feature of an intelligent transponder, combined with an integral processor such that communication to the loop controller of the fact that a particular transponder is in alarm is initiated by the transponder. This is sometimes called polling by exception. This results in lower communications speed while substantially improving control panel response time. Such a feature makes the system less sensitive to line noise and to loop wiring properties; twisted or shielded wire is not required.
Whatever the advantages and benefits of prior art systems, they fundamentally lack an efficient means or arrangement for providing chassis ground fault detection. Although systems have been known in which ground fault detection schemes have been provided, such schemes have not possessed the ability to identify at the control panel the specific location of a field wiring ground fault, thereby to expedite the repair of a ground fault condition.
Accordingly, it is a primary object of the present invention to provide a low cost method or technique that gives specific location identification of a ground fault condition. By specific location, what is meant is that the control panel immediately knows which transponder unit is experiencing a ground fault so that such fault can be remedied in a short time.